Ronnie O'Sullivan, seen by many as the greatest snooker player of all time, succumbed to one of sport's biggest shock defeats when he was beaten 10-8 by amateur James Cahill in the first round of the World Championship on Tuesday.

World No. 1 O'Sullivan has won the world title five times but struggled against the inspired Cahill, who won three qualifying matches to become the first amateur to play at the Crucible Theatre.

Cahill showed few nerves as, leading 5-4 from the first session, he closed out an astonishing victory over 43-year-old O'Sullivan, who has lifted a joint-record 36 ranking trophies in his career.

The 23-year-old was a professional between 2013 and 2017 but returned to amateur status last year. He plans to turn professional again next season.

His win over O'Sullivan -- World Champion in 2001, 2004, 2008, 2012 and 2013 -- was the biggest Crucible shock since seven-time champion Stephen Hendry was beaten 10-7 by Stuart Bingham, then 97th in the world, in 2000.

James Cahill celebrates after beating Ronnie O'Sullivan 10-8 during day four of the 2019 Betfred World Championship at The Crucible, Sheffield. Nigel French/PA Images via Getty Images

"I could barely stand up at the end -- I am not really sure what to say," Cahill told the BBC.

"I have always believed in myself and that I can beat anyone on my day. I want to show what I can do now."

O'Sullivan, who seemed set to go 9-8 ahead before Cahill stole the frame with a superb pressure pot on the final black, said he had struggled throughout.

"I felt shattered, drained, had no energy -- I feel horrendous, to be honest with you," he told the BBC. "All my limbs are feeling heavy.

"I tried to hang in there and do as much as I could, but he has come here and played brilliantly. Fair play to him."

Pundit Steve Davis, a six-time world champion who fell to his own first-round humbling when beaten 10-1 by Tony Knowles as defending champion in 1982, praised Cahill for his approach, saying: "His attitude has been brilliant. He played with rhythm and confidence."